Synthesis and characterization of a pyridine-containing Schiff base oligomer

Article abstract of DOI:10.1007/s11172-006-0497-3

The oligomer of 4-(4-pyridylmethylideneimino)phenol was formed during oxidative polycondensation in aqueous alkaline medium using oxygen as the oxidant. Optimum conditions of the oxidative polycondensation and the main parameters of the process were found

Full text of DOI:10.1007/s11172-006-0497-3

Russian Chemical Bulletin, International Edition, Vol. 55, No. 10, pp. 1852—1855, October, 2006
1852
Synthesis and characterization of a pyridineꢀcontaining Schiff base oligomer
a
H. Ö. Demir,a I. Kaya,b and M. Saçak
ꢀ
aDepartment of Chemistry, Faculty of Science, Ankara University,
Ankara, Turkey
bDepartment of Chemistry, Faculty of Science and Art, Çanakkale Onsekiz Mart University,
17020 Çanakkale, Turkey.
Fax: +90 (286) 218 0533. Eꢀmail: kayaismet@hotmail.com
The oligomer of 4ꢀ(4ꢀpyridylmethylideneimino)phenol was formed during oxidative polyꢀ
condensation in aqueous alkaline medium using oxygen as the oxidant. Optimum conditions of
the oxidative polycondensation and the main parameters of the process were found. The
maximum yield of the oligomer was 52.4%. The product was characterized by elemental
analysis, FTꢀIR spectroscopy, UVꢀVis spectroscopy, and 1Н NMR spectroscopy. The molecuꢀ
lar weight distribution was determined by size exclusion chromatography. Thermogravimetric
and differential thermal analyses were carried out for 4ꢀ(4ꢀpyridylmethylideneimino)phenol
and its oligomer.
Key words: oxygen, oxidative polycondensation, 4ꢀ(4ꢀpyridylmethylideneimino)phenol,
Schiff bases, oligomers.
In recent years, Schiff base polymers containing conꢀ
jugated bonds and active hydroxyl and azomethine groups
have been studied.1 This type of polymers is used in variꢀ
ous fields of technology because of several useful properꢀ
ties, such as paramagnetism, semiconductivity, and resisꢀ
tance to high energies.2,3 Due to this they are used to
prepare composites and graphite materials resistant to high
temperatures, thermostabilizers, epoxide oligomers, block
copolymers, photoresistors, antistatic and flameꢀresistant
materials, and components of electrochemical cells.4—8
Additional useful properties can be profited by the introꢀ
duction of other functional groups into these polymer.
Schiff bases demonstrate antimicrobial activity against
bacteria, yeast, and fungi.9,10 Polyfunctional oligophenols
can be used for purification of industrial waste waters
from heavy metals, which is significant for environmental
protection. Therefore, it seems reasonable to prepare a
polymerꢀbound chelating ligand capable of complexing
with a series of transition metals, which, therefore, can
find a wide use.11
4ꢀ(4ꢀpyridylmethylideneimino)phenol (4ꢀPIP), such as
the temperature, time, and initial concentrations of oxyꢀ
Scheme 1
It is known that oxidative polymerization of phenol
derivatives can produce two types of linkages between
the monomer units: С—С and С—О—С. The reaction
mechanism and coupling selectivity have been studied,12
and three possible mechanisms affecting the С—С couꢀ
pling selectivity have been proposed. The polymerization
of oligophenols13 and polyazomethines14 was found to
proceed regioselectively.
In the present work, we investigated the effects
of different parameters on the oligomerization of
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1785—1788, October, 2006.
1066ꢀ5285/06/5510ꢀ1852 © 2006 Springer Science+Business Media, Inc.

4ꢀ(4ꢀPyridylmethylideneimino)phenol oligomer
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 10, October, 2006
1853
Table 1. Molecular weight distribution of Oꢀ4ꢀPIP under differꢀ
ent synthesis conditions ([4ꢀPIP]0 = [KOH]0 = 0.34 mol L–1
gen and 4ꢀPIP, and determined the optimum reaction
conditions (Scheme 1). Synthesized 4ꢀPIP and its oligoꢀ
mers (Oꢀ4ꢀPIP) were characterized by the IR, UV, and
1Н NMR spectra, elemental and thermogravimetric analyꢀ
ses, and size exclusion chromatography.
)
Entry
v/L h–1 T/°С
t/h
Mn
g mol–1
Mw
PDI
1
2
3
4
5
6
7
0.60
2.40
2.40
2.40
2.40
2.40
9.60
80
70
80
80
80
90
80
2
2
2
4
6
2
2
1718
1155
1915
4799
35286
2085
2315
2775
1688
2816
7245
38315
6900
4172
1.62
1.46
1.47
1.51
1.09
3.31
1.80
Experimental
IR spectra were recorded on a MATTSON 1000 FTꢀIR
spectrophotometer in the range from 400 to 4000 cm–1 in KBr
pellets. UV spectra in the visible range were obtained on a
SHIMADZU UVꢀ1700 instrument in THF. Elemental analysis
was carried out on a LECO CHNS 932 analyzer. NMR spectra
were measured on a Bruker DPX spectrometer (100.6 (1Н) and
400 MHz (13С)) in DMSOꢀd6 relative to Me4Si as internal stanꢀ
dard. Thermal analysis was conducted on a Perkin Elmer Diaꢀ
mond Thermal Analysis instrument. Thermogravimetric and difꢀ
ferential thermal analyses were carried out between 20 and
1000 °С with a heating rate of 10 °C min–1 under nitrogen. The
numberꢀaverage (Mn) and weightꢀaverage (Mw) molecular
weights and polydispersity index (PDI) were determined using
size exclusion chromatography on a SHIMADZU chromatoꢀ
graphy using SGXꢀtype columns (7.7×300 mm, pore size of the
stationary phase 100 Å and 7 nm, DMF—МеОН (4 : 1, v/v)
mixture as eluent, rate 0.4 mL min–1, polystyrene standards,
refractive index detector). 4ꢀAminophenol, 4ꢀpyridinecarbꢀ
aldehyde, inorganic reagents, and solvents (Merck) were used as
purchased. Oxygen was supplied from Muher (Turkey).
4ꢀ(4ꢀPyridylmethylideneimino)phenol. A mixture of 4ꢀpyriꢀ
dinecarbaldehyde (1.0711 g, 0.01 mol) and 4ꢀaminophenol
(1.0913 g, 0.01 mol) in EtOH (25 mL) was refluxed for 4 h. The
precipitated product was filtered off, recrystallized from EtOH,
and dried in a vacuum desiccator. The yield was 93%, m.p.
210 °С. Found (%): C, 72.41; H, 4.11; N, 14.24. C12H10N2O.
Calculated (%): C, 72.71; H, 5.08; N, 14.13. UV, λmax/nm: 228,
240, 260, 288, 310, 340. IR, ν/cm–1: 3452 (O—H), 3064
(C—Harom), 2999, 2881 (C—Haliph), 1600 (CH=N), 1268
Note. v is the oxygen flow rate; t is the reaction time; Mn and Mw
are the numberꢀaverage and weightꢀaverage molecular weights,
respectively; PDI is the polydispersity index.
oven at 105 °С. Found (%): C, 71.73; H, 4.84; N, 13.40.
(C12H8N2O)x. Calculated (%): C, 73.46; H, 4.11; N, 14.28. UV,
λ
max/nm: 233, 252, 266, 290, 313, 346. IR, ν/cm–1: 3412 (O—H),
2810 (C—Harom), 2684, 2613 (C—Haliph), 1600 (CH=N), 1240
(C—O), 1550—1448 (C=Carom), 1166 (C=Narom).
The oligomer is soluble in DMF, THF, DMSO, pyridine,
and aqueous KOH and partially soluble in EtOH, MeOH, and
acetone; however, it is insoluble in AcOEt, benzene, xylene,
hexane, heptane, CHCl3, and CH2Cl2.
Results and Discussion
Study of the synthesis conditions and molecular weight
distribution for Oꢀ4ꢀPIP. The oxidative condensation of
4ꢀPIP was carried out in an aqueous alkaline solution,
and the color of the reaction mixture turned into brown
immediately at the moment of oxygen inlet. The results of
4ꢀPIP oxidative polymerization and the influence of the
reaction parameters are presented in Figs 1—3. At the
initial concentrations [4ꢀPIP]0 = [KOH]0 = 0.34 mol L–1
and the flow rate of oxygen 0.6 L h–1 at 80 °C for 2 h the
yield of Oꢀ4ꢀPIP was 17.4%. The conversion of 4ꢀPIP
increases with an increase in the flow rate of oxygen, and
at 9.6 L h–1 the oligomer yield reaches 33.8% and this
value is further retained. The yield also increases with an
increase in the initial concentration of 4ꢀPIP. Under the
1
(C—O), 1523—1421 (C=Carom), 1165 (C=Narom). H NMR, δ:
6.86 (d, 2 H, H(2), Н(6), J = 8.1 Hz); 7.37 (d, 2 H, H(3), Н(5),
J = 8.1 Hz); 7.80 (d, 2 H, H(3´), H(5´), J = 4.5 Hz); 8.75 (s,
1 H, CH=N); 8.78 (d, 2 H, H(2´), H(6´), J = 4.5 Hz); 9.73 (s,
1 H, OH). 13С NMR, δ: 116.70 (C(2), С(6)); 122.73 (C(3´),
C(5´)); 123.92 (C(3), С(5)); 142.48 (C(4´)); 143.89 (C(4));
151.19 (C(2´), C(6´)); 155.92 (C(1)OH); 158.11 (СН=N).
At ~20 °C 4ꢀPIP is soluble in DMF, THF, DMSO, pyriꢀ
dine, and aqueous KOH. On heating it is also soluble in EtOH,
MeOH, acetone, AcOEt, and partially in CHCl3 and CH2Cl2;
4ꢀPIP is insoluble in benzene, xylene, hexane, and heptane.
Oxidative polycondensation of 4ꢀPIP using oxygen. A soluꢀ
tion of 4ꢀPIP (0.9912 g, 5 mmol) in 10% aqueous KOH (5 mmol)
was placed in a threeꢀnecked roundꢀbottom 50ꢀmL flask
equipped with a thermometer and a reflux condenser, whose
upper part is connected with a tube for oxygen supply. Oxygen
was bubbled through the reaction mixture with the rate from 0.6
to 9.6 L h–1 at temperatures given in Table 1. At the end of the
reaction, the mixture was cooled to ~20 °C and neutralized with
37% HCl (0.410 mL), and the solid product was filtered off and
washed with hot distilled water (3×25 mL) to remove mineral
salts. The unreacted 4ꢀPIP was removed from the oligomeric
product by extraction with AcOEt. The oligomer was dried in an
optimum conditions ([4ꢀPIP]0 = [KOH]0 = 0.34 mol L–1
flow rate of O2 2.4 L h–1, 90 °C, 6 h) the yield of Oꢀ4ꢀPIP
was 52.4%.
,
The chromatogram of the sample, which was obꢀ
tained under the conditions of [4ꢀPIP]0 = [KOH]0
=
0.34 mol L–1, 80 °С, and reaction time 2 h, is shown
in Fig. 4. The Mn, Mw, and PDI values are 2164,
3156 g mol–1, and 1.46, respectively. The chromatogram
contains two peaks. In the material obtained 4% of the
weight belong to lowꢀmolecularꢀweight components
(Mn = 308 g mol–1, Mw = 325 g mol–1, PDI = 1.057), and
96% fall onto highꢀmolecularꢀweight compounds (Mn =
2315 g mol–1, Mw = 4172 g mol–1, PDI = 1.80).

1854
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 10, October, 2006
Demir et al.
Yield (%)
30
20
10
10
14
18
Elution time/min
Fig. 4. Chromatogram of Oꢀ4ꢀPIP synthesized at 80 °С (flow
0
rate of O2 9.6 L h–1, 2 h, [4ꢀPIP]0 = [KOH]0 = 0.34 mol L–1).
0.2
0.4
[O2]/mol L–1
Fig. 1. Plot of the Oꢀ4ꢀPIP yield vs. oxygen flow rate (80 °С, 2 h,
[4ꢀPIP]0 = 0.34 mol L–1).
and reaction time are given in Table 1. It is seen that the
molecular weight of the prepared Oꢀ4ꢀPIP increases with
an increase in these parameters.
Yield (%)
30
Spectral characteristics of 4ꢀPIP and Oꢀ4ꢀPIP. The
UV spectrum of 4ꢀPIP exhibits bands at 228, 240, 260,
288, 310, and 340 nm. The K bands corresponding to the
hydroxyl and azomethine groups are observed at 240 and
310 nm, respectively. The R band of the CH=N group lies
at 340 nm. The spectrum of Oꢀ4ꢀPIP contains bands at
233, 252, 266, 290, 313, and 346 nm. The K bands of the
hydroxyl and azomethine groups lie at 252 and 313 nm,
respectively, and the R band of the CH=N group is obꢀ
served at 346 nm.
20
10
0
0.2
0.4
[4ꢀPIP]0/mol L–1
The IR spectrum of the Oꢀ4ꢀPIP oligomer contains
characteristic bands of the functional groups: phenolic
hydroxyl (3412 cm–1), aromatic CH bonds (3065 cm–1),
azomethine group (1600 cm–1), and double bonds (1550
and 1448 cm–1).
Fig. 2. Plot of the Oꢀ4ꢀPIP yield vs. initial concentration of
4ꢀPIP (80 °C, 2 h, flow rate of O2 2.4 L h–1).
Yield (%)
1
The Н NMR spectrum of Oꢀ4ꢀPIP (Fig. 5) repreꢀ
4
sents a superposition of narrow lines, whose chemical
shift values do not coincide with the position of lines in
50
40
30
20
10
3
2
1
H(3),
H(5)
CH=N
a
H(2),
H(6)
H(2´),
H(3´),
H(5´)
H(6´)
OH
11
10
9
8
7
6
δ
b
0
1.5
3.0
4.5
t/h
Fig. 3. Plots of the Oꢀ4ꢀPIP yield vs. reaction time at 60 (1),
70 (2), 80 (3), and 90 °C (4) ([4ꢀPIP]0 = 0.34 mol L–1, flow rate
of O2 2.4 L h–1).
11
10
9
8
7
6
δ
The plots of the molecular weight distribution in the
synthesized Oꢀ4ꢀPIP vs. oxygen flow rate, temperature,
Fig. 5. 1H NMR spectra of 4ꢀPIP (a) and Oꢀ4ꢀPIP (b).

4ꢀ(4ꢀPyridylmethylideneimino)phenol oligomer
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 10, October, 2006
1855
dm/dt
Heat flow
endo/µV
tion of 4ꢀPIP occurs at 204 °С. For Oꢀ4ꢀPIP, the initial
degradation temperature and the temperatures of the 50
and 57.50% weight loss are 194, 815, and 1000 °С, reꢀ
spectively. The presence of water in Oꢀ4ꢀPIP is also seen
in the thermogravimetric curves (see Fig. 7): the 2% weight
loss is observed in a temperature interval of 100—170 °С,
which can be attributed to crystallization (50—150 °С)
and loss of coordination water (150—200 °С).
Thus, the synthesized oligomer Oꢀ4ꢀPIP exhibits sufꢀ
ficient thermal stability. The results obtained can be used
for the development of new functionally substituted polyꢀ
meric materials.
(% min–1
)
m (%)
1
100
80
60
40
20
0
–1
–2
–3
–4
–40
2
–20
0
3
20
200
400
600
800
T/°C
Fig. 6. TGA (1), DTG (2), and DTA (3) curves for 4ꢀPIP.
Curve 1, f(m); curves 2 and 3, f(dm/dt).
This work was financially supported by the Ankara
University Research Foundation (Project No. 2002ꢀ
07.05.069).
dm/dt
(% min–1
Heat flow
endo/µV
)
m (%)
100
0
–0.2
–0.4
–0.6
–0.8
–1.0
References
0
1
80
2
1. A. G. ElꢀShekeil, F. A. AlꢀYusufy, and S. Saknidy, Polym.
Int., 1997, 42, 39.
2. S. C. Suh and S. C. Shim, Synth. Met., 2000, 114, 91.
3. B. A. Mamedov, Yu. A. Vidadi, D. N. Alieva, and A. V.
Ragimov, Polym. Int., 1997, 43, 126.
4. M. Grigoras and C. O. Catanescu, J. Macromol. Sci., Part C:
Polym. Rev., 2004, 44, 131.
5. A. V. Ragimov, B. A. Mamedov, and S. G. Gasanova, Polym.
Int., 1997, 43, 343.
6. R. H. Baughman, J. L. Bredas, R. R. Chance, R. L.
Elsenbaumer, and L. W. Shacklette, Chem. Rev., 1982,
82, 209.
7. F. R. Diaz, J. Moreno, L. H. Tagle, G. A. East, and D. Radic,
Synth. Met., 1999, 100, 187.
8. K. I. Aly and A. A. Khalaf, J. Appl. Polym. Sci., 2000,
77, 1218.
9. I. Kaya, A. R. Vilayetoglu, and H. Topak, J. Appl. Polym.
Sci., 2002, 85, 2004.
20
40
60
40
3
20
60
200
400
600
800
T/°C
Fig. 7. TGA (1), DTG (2), and DTA (3) curves for Oꢀ4ꢀPIP.
Curves 1 and 2, f(m); curve 3, f(dm/dt).
the initial 4ꢀPIP, and broad signals. The narrow lines
belong, most likely, to lowꢀmolecularꢀweight compoꢀ
nents, and the broad lines are from highꢀmolecularꢀweight
components. This spectrum provides no clear evidence
for the type of linkage of monomeric units; however,
published data15 suggest that coupling occurs at the
orthoꢀpositions of the phenol fragments (see Scheme 1).
Note that the polymers with the azomethine structure
containing 1,4ꢀphenylene and 1,5ꢀnaphthylene units have
been synthesized previously16 by the polycondensation of
dialdehydes with diamines.
Thermal analysis of 4ꢀPIP and Oꢀ4ꢀPIP. The thermal
analysis curves of the monomer and oligomer are shown
in Figs 6 and 7, respectively. For 4ꢀPIP the initial degraꢀ
dation temperature and the temperatures of the 50 and
66.32% weight loss are 195, 460, and 1000 °С, respecꢀ
tively. The thermal decomposition of the monomer proꢀ
ceeds in two stages. At the first stage, 14% of the initial
weight are lost in the temperature interval from 170
to 250 °С. The weight loss at the second stage is the
highest (46%) and occurs in the 250—600 °C interval.
According to the DTA data, the exothermic decomposiꢀ
10. I. Kaya, H. Ö. Demir, and A. R. Vilayetoglu, Synth. Met.,
2002, 126, 183.
11. I. Kaya and M. Gül, Eur. Polym. J., 2002, 40, 2025.
12. S. Kobayashi and H. Higashimura, Prog. Polym. Sci., 2003,
28, 1015.
13. M. N. Patel and S. H. Patil, J. Macromol. Sci. Chem., 1981,
A16, 1429.
14. I. Kaya, J. Polym. Res., 2004, 11, 175.
15. M. S. Ayyagari, K. A. Marx, S. K. Tripathy, J. A. Akkara,
and D. L. Kaplan, Macromolecules, 1995, 28, 5192.
16. O. Catanescu, M. Grigoras, G. Colotin, A. Dobreanu,
N. Hurduc, and C. I. Simionescu, Eur. Polym. J., 2001,
37, 2213.
Received February 28, 2006;
in revised form August 29, 2006